1. Field of the Invention
The present invention generally relates to light-emitting diodes and light-receiving devices, and particularly relates to light-emitting diodes, light-receiving devices, and arrayed light sources which can be used as light sources or light-receiving devices for optical communication using optical fibers, for optical fiber illumination, for optical printers printing on a photosensitive material, etc.
2. Description of the Prior Art
Various researches on light-emitting diodes and light-emitting diode arrays have been conducted, aiming for the application of those devices as light sources in optical communication, optical printers employing the electronic photograph printing method, etc.
As a light source for printing purposes such as in optical printers, a light-emitting diode array has such advantages as having no moving part and comprising a small number of components. This leads to a miniaturization of an optical printer head.
Other advantages of such use include a high contrast realized by a high extinction rate of a light-emitting diode of the self-emitting type, an elongation of a chip length realized by a continuation of a chip, a high speed print realized by boosting the output of the light-emitting diode, etc.
The use of light-emitting diodes as a light source for optical communication is typically limited in optical communications of the short-to-middle distance range. The light-emitting diodes used as light sources for optical communication are required to have a high efficiency of light emission, a high efficiency in taking light out of the crystals, and a high efficiency in leading light into optical fibers.
The light-emitting diodes employed for such a use include light-emitting diodes of the surface-emitting type, light-emitting diodes of the edge-emitting type, etc.
FIG. 1 shows a basic structure of a light-emitting diode of the surface-emitting type which is reported in Proceedings of the 1980 IEIC (The Institute of Electronics, Information, and Communication Engineers) Conference, Communication Field, No.1, pp211.
In such light-emitting diodes as those, electrodes 34 are provided around or on both sides of light-emitting parts 33, in order to make the intensity of an optical output homogeneous across the light-emitting surface.
Since the light-emitting parts for generating optical output and the electrodes are provided on the same surface, a width of one element is the sum of each width of the light-emitting part, the electrode, and a space required for separating each element. Thus, implementing the light-emitting parts in such a high density as 600 dpi (dots per inch) is extremely difficult.
Also, since the light-emitting diodes of the surface-emitting type have a wide emitting surface, light emitted therefrom has a beam pattern of almost no directivity despite a high intensity. Thus, an efficiency of optical coupling with lenses or optical fibers is low, meaning that the use of the light emitted from the light-emitting diode is not so efficient.
FIG. 2 shows an example of a light-emitting diode of the edge-emitting type which is disclosed in the Japanese Laid-Open Patent Application Number 60-32373. In this example, a plurality of light-emitting parts 35 are formed within a piled-layer structure of a substrate. The light-emitting parts 35 are separated physically and electrically from each other by separation grooves 36 formed in a direction perpendicular to the top surface of the substrate.
In the light-emitting diode such as this, the light-emitting parts 35 and the electrodes 37 and 38 are not provided on the same plane. A width of one element is thus the sum of a width of the light-emitting part and a space required for separating each element. Thus, it is theoretically possible to form the light-emitting parts in a density higher than 600 dpi.
Accordingly, it can be said that an array of light-emitting diodes of the edge-emitting type is more suitable than that of the surface-emitting type for use as a light-emitting diode array for a high density printer. In the light-emitting diodes of the edge-emitting type, however, light is emitted from a lateral surface of a substrate, which has a relatively narrow area. Thus, the intensity of the light emission is not as high as that of the light-emitting diodes of the surface-emitting type.
Light-emitting diodes of the edge-emitting type has almost no directivity within a plane parallel to a substrate surface. Within a plane perpendicular to the substrate surface, however, there is a directivity to some extent, having a shape of an elongated ellipse. Thus, an efficiency of optical coupling with optical fibers and the like is higher than that of the surface-emitting type. Also, in comparison with the surface-emitting type, a more efficient use can be realized for light emitted from light-emitting diodes.
There are various methods proposed for calibrating an optical output of edge-emitting devices. FIG. 3 and FIG. 4 show methods devised for such a purpose which are disclosed in the Japanese Laid-Open Patent Application Number 62-15878. In FIG. 3 and FIG. 4, light from light-emitting device 30 or 10 is monitored by light-receiving device 32 or 11. The output of light emission can be calibrated by adjusting the amount of a current flowing into the light-emitting device 30 or 10.
In the light-emitting diodes of the edge-emitting type of the prior art, however, the light-emitting surface is formed by such methods as cleavage so that this surface ends up having a single plane shape perpendicular to the substrate. Thus, light emitted from such a surface has a directivity to some extent within a plane perpendicular to a light-emitting layer, but has almost no directivity within a plane parallel to the light-emitting layer.
Accordingly, there is a large amount of light which is emitted from the surface at an angle larger than an angular aperture of a lens or an optical fiber. Such light cannot be optically coupled with the lens or the optical fiber.
This means that an optical coupling efficiency within a plane parallel to the light-emitting layer is low because there is almost no directivity within this plane.
When monitoring an optical output by light-receiving devices which have a light-receiving surface perpendicular to a substrate, the area of the light-receiving surface decreases as a dot density increases. Thus, a sufficient amount of light is difficult to obtain.
A light-receiving device which has a slanted light-receiving layer as shown in FIG. 3 makes a device forming process more complex because an additional substrate processing process is required before a process of crystal growth.
Also, when etching electrodes and wires made of Au and the like in patterning the devices of the prior art, a plurality of etching agents are required in order to form a layered structure of wiring. Even when using such a method as lift-off, a process of forming the devices of the prior art is complex.
Accordingly, there is a need in the field of optical devices for the following optical devices which are highly efficient in use of light: a light-emitting diode of the edge-emitting type, an arrayed light source, a light-receiving device of the lateral-surface-receiving type, a light-emitting/receiving device, and a light source of a light-emitting diode array.
There is another need for a light-emitting/receiving device which can monitor light emission from light-emitting device of the edge-emitting type by using a light-receiving device of the lateral-surface-receiving type capable of receiving a large amount of light.
There is still another need for a light source of a light-emitting diode array which has an optical output calibration mechanism and can be formed in a process simpler than that of the prior art.
There is yet another need for an arrayed light source comprised of the semiconductor devices described above which can be formed in a device forming process simplified by using materials easy to pattern electrodes.